Influence of toroidal rotation on plasma response to external RMP fields in tokamak

Abstract

The type-I edge localized mode (ELM) is a critical event associated with magneto-hydrodynamic(MHD) instabilities occurring in tokamak high-confinement (H-mode) discharges, that leads to huge heat loads on the plasma phasing components (PFC) and may result in material damages. It is important to effectively control large ELMs, in order to ensure safe operation of the future reactor-scale devices such as ITER and DEMO. Resonant magnetic perturbation (RMP) has been experimentally demonstrated to be a mature and robust technique for controlling ELMs. A set of parameters, such as the edge safety factor, the plasma flow, the RMP coil geometry and the spectrum of the applied external field, have been found to play important roles in controlling ELMs by RMP. Furthermore, the plasma pressure is known to affect the plasma response to the RMP field, in particular near the no-wall beta limit. This is because high plasma pressure drives the resonant field amplification of the external field by the plasma response. The ITER 10 MA steady state scenario will be operated near the no-wall stability limit. The new tokamak device HL-2M will also operate in the relatively high-beta regimes. On the other hand, more investigations are still needed to understand the influence of toroidal flow on the high-beta plasma response. This work employs a single fluid toroidal model to compute the plasma RMP response in HL-2M, emphasizing on the roles of two key physical quantities: the plasma resistivity and the toroidal rotation. The former allows penetration of the external RMP field into the plasma, while the latter mainly provides screening effect on the resonant field component. More specifically, the MARS-F code is utilized to study the plasma response to the externally applied <i>n</i> =1 ( n is the toroidal mode number) RMP field for high-beta HL-2M discharges, while varying the plasma toroidal rotation profile. The plasma response is found to (i) substantially modify the poloidal spectrum of the applied vacuum RMP field, (ii) change the amplitude of the resonant radial field amplitude near the plasma edge, and (iii) affect optimal current phasing between the two rows of RMP coils on HL-2M. A sufficiently slow toroidal flow near the plasma edge amplifies the radial field at rational surfaces associated with the perturbation. Since the latter serves as a reliable indicator for controlling the type-I edge localized mode (Type-I ELM) by RMP, varying rotation profile near the plasma edge offers a promising approach to optimize ELM control.